The present invention relates to a liquid crystal display and, more particularly, to a lighting device using a prism lens for converging back light rays and for use in a back lighting device for liquid crystal displays, and a transmission type liquid crystal display for use in a portable type personal computer, word processor and liquid crystal television set and so on, and a back lighting device suitable for use in a transmission type liquid crystal display such as a portable type personal computer, a word processor, a liquid crystal television set etc., and more particularly, to a back lighting device using a light guiding plate.
Liquid crystal displays (LCD) do not radiate light and are required to be illuminated when they are being used. Accordingly, liquid crystal displays are each provided with a back light for ensuring the necessary luminosity. A back light is composed of a light source and a light diffusing element. Lighting from a light source such as an incandescent lamp (point-light source), a fluorescent lamp (line-light source) and the like is converted from a light diffusing element to surface light for illuminating a liquid crystal display from the back. There are many kinds of proposed liquid crystal displays provided with back lights.
For example, the publication of the unexamined patent application JP,A,2-77726 discloses a liquid crystal display which has an aspherical condenser lens placed between a point light source and a liquid crystal panel and has an aspherical Fresnel lens.
The publication of the unexamined patent application JP,A,61-15104 discloses a liquid crystal display which has a light diffuser composed of a plurality of photoconductors disposed stepwise between a line light source and a diffusing plate having triangle-pole prisms disposed thereon.
The publication of the unexamined utility model application JP,U,2-62417 discloses a liquid crystal display which has a transparent plate between the light source and the diffusing plate and the transparent plate has a plurality of grooves placed thereon to function as a prism opposite the light source.
As one of the means to increase the luminosity of the display light a method is proposed for increasing the luminosity of the display light in a specified direction by converging diffused light of the back lighting device by the use of a prism lens.
As mentioned above, the application of a conventional prism lens may increase the luminosity of a liquid crystal (LC) display in a specified direction but it produces a view angle with no quantity of light. This may impair the quality of the image on the LC display. Accordingly, a diffusing sheet is usually used between a prism lens and a liquid crystal (LC) panel. This means that light rays are converged and diffused again. Therefore the intended effect of the use of the prism lens cannot be attained. It is possible to omit the diffusing sheet by increasing the vertex angle of a prism lens to the level at which the effect of a non-lighting quantity angle may be negligible. However, this may have little effect on the prism lens since the prism has an increased vertex angle that has decreased the converging power.
The conventional liquid crystal display produces such a problem that external light may be reflected by front and rear surfaces of a transparent protection plate, a front surface of and in a liquid crystal panel, thereby its image visibility is impaired.
The conventional liquid crystal displays have a limit in achieving reduction of their thickness and, at the same time, the increase of the brightness of their display screens. For instance, it is difficult to reduce the thickness of the liquid crystal display using a direct bottom-reflecting type back light. An attempt to increase luminance of the lamp by increasing current is accompanied by heating of the lamp, which leads to the damaging of the IC chips for the horizontal and vertical drivers disposed at the periphery of the display screen. It was difficult to satisfy both above-mentioned requirements at the same time.
One of the disadvantages of the conventional light-guiding type back lighting device is that separate components may expand to create uneven spacing therebetween or be displaced during the assembly thereby causing uneven luminosity by the back light rays during the operation of the devices. In particular, in the case of assembling very thin components, e.g., 0.2 mm thick converging, diffusing and reflecting plates, extreme care must be taken to prevent the expansion and displacement and also to avoid the erroneous positioning of the components and of damage from dirt on their surfaces that may cause dark and luminescent spots on an image on the display screen of the liquid crystal panel. In addition, any conventional back light device may leak light rays through a gap between its lamp and light guiding plate and the device and its holder that causes the weaker luminosity of an image on the liquid crystal panel.
The other disadvantage of the prior art is as follows: liquid crystal panels have a view angle characteristic that is particularly narrow in its vertical direction. Therefore, even if back light rays are applied uniformly at all visual angles to a liquid crystal display panel, the picture image quality of the display can be scarcely improved because of the very narrow viewing angle of the panel in a vertical direction.
The present invention aims to achieve an increase in luminosity and to decrease the reflection of a liquid crystal display provided with an edge, light-type back lighting unit.
Technical subjects are (1) increasing the luminosity of a back lighting unit and (2) reducing the reflection of a liquid crystal panel.
Regarding the subject (1), the present invention proposes a back light that comprises a lamp (light source), a light guiding plate, a diffusing plate and a converging plate. On the other hand, Japanese publications of unexamined application JP,A,61-15104 and JP,U,2-62417 disclose the relevant prior art. The former prior art is similar to the present invention in the application of an edge light-type back lighting unit, but said unit is composed of a light source, a light-guiding plate and a double-layered diffusing plate to eliminate the unevenness of the light for illuminating a liquid crystal panel. The latter prior art used a direct bottom-reflecting type back light which was composed of a light source, a converging plate and a diffusing plate through which light rays illuminated the liquid crystal panel while the present back lighting device, according to the present invention, is to illuminate the liquid crystal panel with back light rays through the use of a converging plate.
Essentially, the former prior art was intended to prevent uneven luminosity within the display screen only and the latter prior art was intended to increase (to the maximum) the luminosity of a specified position within the display screen not taking into account the uneven luminosity therein. The present invention proposes to increase the front axial luminosity of the display screen and at the same time to prevent the uneven luminosity thereon as much as possible.
Regarding the subject (2), the present invention proposes that a liquid crystal panel and a transparent protection plate having an AR coat (reflecting coat) applied on its front surface are stuck together with adhesive.
The publication of unexamined patent application, JP,A,3-188420 discloses a device which applies a combination of a circular light-deflecting disc and a 1/4-phasing substrate to suppress light reflection in and at surface of a panel and at the rear surface of a protection plate. This combination, however, requires provision of an air gap between the panel and the transparent protection plate. The present invention aims to eliminate the light reflection by surface of the liquid crystal panel and by the rear surface of the transparent protection plate by sticking the protection plate and the panel together with adhesive.